1. Description of the Related Art
The present invention relates to magnetic fluid seal systems.
2. Description of the Known Technology
Magnetic fluid seals generally include a rotatable shaft and a unitized sub-assembly of ring magnets and pole rings. Dimensions are controlled to produce small annular gaps between pole ring tips and the shaft surface. Strong magnetic fields exist in these gaps. A small amount of ferrofluid is added at each gap and is held by the field as liquid rings in the gaps, with gas-filled spaces confined between adjacent rings. The number of magnets and annular gaps may vary and the pole rings may, in fact, be formed as a single pole piece. Whatever the detailed design may be, all such devices perform their sealing function as described in the next paragraph.
If pressure is the same on both sides of a fluid ring, the fluid assumes an equilibrium position determined by the strength and configuration of the local magnetic field. Any difference in pressure from one side of a fluid ring to the other tends to displace the fluid axially from its equilibrium position. Displacing a ring from its equilibrium position leads to a net axial force that opposes the pressure difference that produced the displacement. If the pressure difference becomes large enough, the liquid ring bursts open, and gas flows from one side of the gap to the other. The pressure at which the ring bursts is called the “pressure capacity” of the ring. The pressure capacity of a multi-stage device (i.e. multiple liquid rings arranged in series on the same shaft) is the sum of the pressure capacities of individual stages. As fluid seals are utilized in environments of significant pressure difference, increasing the pressure capacity of ferrofluid seals is highly desirable.